In transmitters and receivers, in-phase (I) and quadrature-phase (Q) phase signals become imbalanced due to analog circuits within a transmitter or receiver. For example, analog circuits may include demodulators, low pass filters and operational amplifiers. An analog circuit in a receiving path may have a zero intermediate frequency (IF) design that produces imbalanced I and Q signals.
IQ imbalance may be caused by a phase offset in demodulator cosine and sine signals, a gain difference between an I-path and a Q-path in the low pass filters and operational amplifiers, and a path length (delay) difference between the I-path and Q-path.
For example, quadrature carriers in an analog modulator do not have exactly the same amplitudes and an exact phase difference of 90 degrees. This causes cross-talk between the I and Q paths, which is referred to as IQ imbalance.
The nature of the IQ imbalance is that an amount of the I signal spills into the Q signal and an amount of the Q signal spells into the I signal. The frequency response of the IQ imbalanced signal has not only the original signal but also an image, at the negative frequency. For example, a 5 MHz tone has an image signal at −5 MHz, with lower amplitude. A wideband signal has a wideband image with negative frequencies. If the frequency band of the wideband signal overlaps with its image, the wideband signal and images are superimposed in the overlapped frequencies.